The present invention relates to compositions for forming soil preventing coatings.
The submerged portions of ships and marine structures are usually coated with soil preventing coating compositions so as to prevent corrosion or decreases in the speed of navigation of ships due to the deposition of marine organisms such as barnacles, polyzoa, ascidians and seaweed. Such soil preventing coating compositions are applied also to culture nets to preclude lethal damage to fishes and shellfishes due to the deposition of marine organisms. These compositions include those comprising a polymer, rosin, plasticizer and like vehicle components and a soil preventing agent, and those comprising a polymer of organotin-containing unsaturated monomer or a copolymer of such monomers as disclosed, for example, in Examined Japanese Patent Publications 21426/1965, 9579/1969 and 12049/1976.
The former compositions exhibit a soil preventing effect due to the dissolving out of the rosin and the soil preventing agent contained in the coating formed by the composition, whereas when the amount of insolubles of the coating increases during a prolonged period of submersion, the coating becomes rough-surfaced and less effective for preventing deposition of marine organisms. The latter compositions form a coating containing an organotin compound which is introduced into the polymer through an ester bond. When the coating is degraded by seawater, the ester bond undergoes hydrolysis, releasing the organotin compound and producing a carboxyl-containing polymer. This polymer dissolves in seawater, continuously refreshing the coating surface as exposed to the water to maintain a soil preventing effect. However, the coating contains the organotin compound generally in a large amount so as to effectively exhibit the soil preventing effect, hence the drawback of exerting an adverse influence on fishes and shellfishes.
To overcome the drawbacks described, the present applicant has proposed a method of forming soil preventing coatings with use of a composition wherein a resin having an acid anhydride group serves as a vehicle component as disclosed in Unexamined Japanese Patent Publication 99567/1990. More specifically, the resin used in this coating composition is a copolymer having a cyclic acid anhydride group in the molecule and is prepared by reacting an acid anhydride, such as itaconic anhydride or maleic anhydride, with other unsaturated monomer for radical copolymerization. The coating formed by the copolymer forms two carboxyl groups on hydrolysis and becomes soluble in water, whereas if the copolymer has a large amount of acid anhydride groups, the coating fails to retain the soil preventing effect satisfactorily over a prolonged period of time. Conversely, if the copolymer has an insufficient number of acid anhydride groups, the coating is unable to fully exhibit the soil preventing effect even for a short period of time. Thus, the proposed composition also has a drawback.